The PSA method has been described as being adiabatic, that is to say a process which occurs without loss or gain of heat. Accordingly, the end-to-end temperature of each adsorbent bed used to perform the PSA process is predicted by theory to be uniform. Each active particle of adsorbent adsorbs heat and rises in temperature during adsorption. During desorption, the particle releases heat and hence falls in temperature. It is therefore to be expected that at steady state, the quantity of fluid adsorbed by a particle equals the quantity desorbed and hence the quantity of heat generated during the adsorption equals the loss of heat during desorption. Since each particle can be identified as a separate adiabatic zone, it might be predicted that temperature differentials within a PSA bed should not occur.
In practice, however, as is well known, such temperature differentials are created, particularly when using a bed of zeolite molecular sieve to separate air, the sieve adsorbing nitrogen in preference to oxygen. At steady state, which is typically established after say 24 or 36 hours of continuous operation, there is found to be a temperature gradient, with the temperature of the bed along its longitudinal axis falling from the bottom or feed gas end of the bed to a minimum at a point relative near that end and then rising again to a maximum which is at or near to the top end of the bed. A minimum temperature of less than minus 50.degree. C. and a maximum temperature of at least 30.degree. C. (i.e. above ambient temperature) have been observed. The conclusion to be drawn is that in a bottom section of the bed there is a net loss of heat in each operating cycle, while in a top section there is a net generation of heat. The creation of the temperature differential within the bed can adversely affect the performance of the PSA method. In particular, in the example of the separation of air using a zeolite sieve, although the adsorption capacity of the sieve increases with decreasing temperature, adverse kinetic effects occur so as to reduce its overall performance. Indeed, a sieve optimised for room temperature does not perform as well at lower temperatures.
Various methods have been proposed to reduce the magnitude of an internal temperature differential that is created within a PSA bed. First, it has been proposed that the gas mixture to be separated be heated by external means. Such a method is described in GB-A-1 530 603. In addition, or alternatively, it has been proposed in GB-A- 1 530 604 to transfer heat to the lower temperature portion of the bed from both the top and bottom of the bed by metal-to-adsorbent conduction of heat. To this end, metal rods extend vertically upwards through the bed from its bottom to near its top. The rods may, for example, be of copper or other heat conductive metal.